The present invention relates to an accelerate test method for evaluating the endurance characteristic of a ferroelectric memory device comprising a capacitor element having a ferroelectric film.
The endurance characteristic of a ferroelectric memory device comprising a capacitor element having a ferroelectric film (ferroelectric capacitor) is determined by a polarization inversion fatigue characteristic in which the remanence of the ferroelectric film decreases with the increase of the number of the occurrences of polarization inversion in the ferroelectric film. For an accelerated test for evaluating the endurance characteristic, a method using only a voltage stress as an accelerated condition has been used conventionally.
Referring to FIGS. 5 and 6, a description will be given herein below to a first conventional embodiment (Japanese Laid-Open Patent Publication No. HEI 11-174026) in which the endurance characteristic of a ferroelectric memory device is evaluated by an accelerated test method using only a voltage stress as an accelerated condition.
As shown in FIG. 5, a first regression line at a first polarization inversion voltage V1 is determined by measuring remanences while gradually increasing the number N of the occurrences of polarization inversion at the first polarization inversion voltage V1 in a ferroelectric capacitor and plotting the number N of the occurrences of polarization inversion as abscissa and the decrease rate of the remanence as ordinate. Then, the foregoing operation is repeatedly performed even at a second polarization inversion voltage V2 and a third polarization inversion voltage V3, whereby a second regression line at the second polarization inversion voltage V2 and a third regression line at the third polarization inversion voltage V3 are obtained.
Next, first, second, and third numbers NL1, NL2, and NL3 of the occurrences of polarization inversion when the remanence reaches a specified decrease rate relative to the initial value thereof are obtained based on the first, second, and third regression lines.
Next, a regression line (hereinafter referred to as a linear model) shown in FIG. 6 which is represented by logNL=C−αV (where each of C and α is a constant) is determined from the first, second, and third polarization inversion voltages V1, V2, and V3 and from the first, second, and third numbers NL1, NL2, and NL3 of the occurrences of polarization inversion.
Next, the number of the occurrences of polarization inversion indicative of an expected lifetime (e.g., the number NLb of the occurrences of polarization inversion) under an accelerated condition (e.g., polarization inversion voltage Vb) which corresponds to the number of the occurrences of polarization inversion (e.g., the number NLa of the occurrences of polarization inversion) indicative of an expected lifetime under an actual operating condition (e.g., polarization inversion voltage Va) is determined from the linear model shown in FIG. 6.
By thus conducting an accelerated test on the ferroelectric memory device under accelerated conditions (the polarization inversion voltage Vb and the number NLb of the occurrences of polarization inversion), the endurance characteristic thereof under actual operating conditions (the polarization inversion voltage Va and the number NLa of the occurrences of polarization inversion) is evaluated in a short period of time.
Referring to FIG. 7, a description will be given herein below to a second conventional embodiment (see Jpn. J. AppI. Phys., Vol. 33 (1994) pp. 3996–4002) in which the endurance characteristic of a ferroelectric memory device is evaluated by an accelerated test method using only a voltage stress as an accelerated condition.
FIG. 7 shows a relationship between the logarithm of the number of the occurrences of polarization inversion and the reciprocal of a polarization inversion voltage.
First, the remanence is measured in the ferroelectric capacitor while gradually increasing the number Lc of the occurrences of polarization inversion at a specified polarization inversion voltage so that the number Lc of the occurrences of polarization inversion when the measured remanence reaches a specified decrease rate relative to the initial value of the remanence is determined. By repeatedly performing the foregoing sequence of processes for determining the number Lc of the occurrences of polarization inversion a plurality of times while changing the specified polarization inversion voltage, the number Lc of the occurrences of polarization inversion at each of the specified polarization inversion voltages is determined.
Next, the regression line (hereinafter referred to as a reciprocal model) shown in FIG. 7 which is represented by logLc=B+d/V (where each of B and d is a constant) is determined by plotting the number (Lc) of the occurrences of polarization inversion as ordinate and the reciprocal of the specified polarization inversion voltage (V) as abscissa.
Next, from the reciprocal model of FIG. 7, the number of the occurrences of polarization inversion (e.g., the number Lce of the occurrences of polarization inversion) indicative of an expected lifetime under an accelerated condition (e.g., a polarization inversion voltage Ve) which corresponds to the number of the occurrences of polarization inversion (e.g., the number Lcd of the occurrences of polarization inversion) indicative of an expected lifetime under an actual operating condition (e.g., a polarization inversion voltage Vd) is determined.
By thus conducting an accelerated test on the ferroelectric memory device under the accelerated conditions (the polarization inversion voltage Ve and the number Lce of the occurrences of polarization inversion), the endurance characteristic thereof under the actual operating conditions (the polarization inversion voltage Vd and the number Lcd of the occurrences of polarization inversion) is evaluated.
As a result of examining which one of the first conventional embodiment (linear model) and the second conventional embodiment (reciprocal model) each evaluating the endurance characteristic of a ferroelectric memory device by an accelerated test method using only the voltage stress as an accelerated condition performs an accelerated test which provides the endurance characteristic with a higher fittability to the actual endurance characteristic, the present inventors have concluded that the accelerated test using the reciprocal model provides the endurance characteristic with a higher fittability.
As a result of repeatedly performing an accelerated test on the endurance characteristic of a ferroelectric memory device by using the reciprocal model using only the voltage stress as an accelerated condition, however, the present inventors have found that even the accelerated test using the reciprocal model is unsatisfactory in terms of the fittability to the actual endurance characteristic of a ferroelectric memory device.